Analysing Substances

In your chemistry lessons you will have learned some methods in KS3 including chromatography, the flame test for metals and the iodine test for starch, but during your GCSE you will use several more. For your exams and practical work, you need to learn these and know how to interpret the results.

The flame test is used to find out what metal ion (cation) is present in a compound. It works because different metal ions burn with different colours. The key to a good flame test is to get the equipment as clean as possible so that the colour can be seen. Even the tiniest quantity of sodium ions give a very strong orange/yellow flame and if the flame test wire is not thoroughly cleaned, all you will see is the sodium colour.

The flame test covers some of the most common cations. Precipitation reactions using sodium hydroxide can identify some that are not detectable using the flame test. The precipitate formed by aluminium will re-dissolve if you continue to add sodium hydroxide whereas those of calcium and magnesium do not. The colour of the precipitate can give you some information too, for example, a brown precipitate indicates that the cation is iron III.

The anion (non metal or group of non metals) can be discovered using silver nitrate solution (for halides), barium chloride (for sulfates) or acid and limewater (for carbonates).

Analysing substances needs to be carried out safely and methodically. Start with the flame test and work through the other tests until you have a result. Negative test results are almost as useful as positive results as they can eliminate certain cations and anions and narrow down the range of tests needed.